In gearing-up or gearing-down between two axes, a gear able to realize a variable transmission velocity ratio is called the variable-ratio gear. The variable-ratio gear is mainly used in transmission occasions with special requirements, for example, in aspects of stepless speed regulation in automobile, oval gear flowmeter, and manipulator with variable transmission ratio.
A gear often used for transmission with variable transmission ratio is mainly a non-circular gear, since a non-circular gear mechanism may realize a special movement and function operation, such as swing, indexing, and varying speed, and in the meantime, the non-circular gear may be designed according to transmission ratio function. At present, the non-circular gear used for transmission with variable speed ratio refers to a non-cylindrical gear, and a non-bevel gear. Elliptic bevel gear is a typical non-bevel gear, in named for its large end node being a spherical ellipse. Orthogonal non-circular surface gear pair is a new-style variable-ratio gear transmission, and it has advantages of the non-circular gear, non-bevel gear and face gear, with its design and processing being more simple than the non-circular gear with variable transmission ratio. In addition, variable transmission ratio may be realized by a planetary gear set. Continuous variable magnetic gear comprises a three-phase winding stator and three concentric rotors. By controlling the speed of a central rotor, the transmission ratio between an input rotor and an output rotor can vary.
Line gear is a new-type gear utilizing a space curve meshing theory in the place of a traditional space curve surface meshing theory, which is mainly applied to micro transmission field, and has advantages such as small dimension, large transmission ratio, and easy manufacturing. At present, the line gear pair can be applied to transmissions on two vertical axes, two intersecting axes and two skewed axes, and its researches such as design equation, contact ratio, strength criterion, micro transmission and manufacturing field have tended to be perfect.
In a micro mechanical device with limited dimension, sometimes there is complicated transmission requirement of periodically variable transmission ratio, which is hard to realize by controlling, while traditional non-circular gear is not suitable for micro dimension.